This is one reason why women used to put belladonna (atropine) drops in their eyes. It dilates the pupils, and when the user would speak to a man he'd unconsciously respond to the signal of sexual arousal sent by her dilated eyes.

Or as I said to my husband after my last visit to the ophthalmologist, "I've either been refracted or I'm very glad to see you...or both."

They've done studies showing the same pictures of people with pupils dialated and not-dialated to test subjects. The ones with dialation were scored on average as more attractive than the same picture undialated. So the conclusion is that people may be unconsciously aware of this signal of sexual attraction and respond in kind.

Anyway, when romance novels describe a gaze darkening, I always figure that's what's happening. Your eyes do get darker because the pupils enlargen- and it doesn't go unnoticed!